1. Field of the Invention
The present invention relates to a method of driving an ink-jet printhead. More particularly, the present invention relates to a method of driving an ink-jet printhead that is able to improve a tendency of an ink droplet ejected from the ink-jet printhead to travel straight, i.e., perpendicular to an upper surface of the ink-jet printhead, by applying a main pulse and then a post pulse to a heater. The main pulse has sufficient energy to eject an ink droplet and the post pulse has insufficient energy to eject an ink droplet but sufficient energy to generate a bubble in ink. The post pulse is applied to the heater before a meniscus, which is generated by the main pulse, on a surface of ink returns to a stable state.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is an electro-thermal transducer ink-jet printhead (bubble-jet type) in which a heat source is employed to form and expand a bubble in ink to cause an ink droplet to be ejected due to the expansive force of the formed bubble. A second type is a electromechanical transducer ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink and a change in ink volume due to a deformation of a piezoelectric element.
An ink droplet ejection mechanism of a thermal ink-jet printhead will now be described in detail. When a pulse current is applied to a heater, which includes a heating resistor, the heater generates heat and ink near the heater is instantaneously heated to approximately 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated. Expanding bubbles exert pressure on ink filling an ink chamber. As a result, ink around a nozzle is ejected from the ink chamber in the form of a droplet through the nozzle.
Thermal driving methods include a top-shooting method, a side-shooting method, and a back-shooting method depending on the direction in which the ink droplet is ejected and the direction in which a bubbles expands. The top-shooting method is a method in which the growth direction of a bubble is the same as the ejection direction of an ink droplet. The side-shooting method is a method in which the growth direction of a bubble is perpendicular to the ejection direction of an ink droplet. The back-shooting method is a method in which the growth direction of a bubble is opposite to the ejection direction of an ink droplet.
An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after ink has been ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase a driving frequency.
FIG. 1 illustrates an exploded perspective view of a conventional ink-jet printhead using a top-shooting method. FIG. 2 illustrates a cross-sectional view of a vertical structure of the conventional ink-jet printhead of FIG. 1.
Referring to FIG. 1, the ink-jet printhead includes a base plate 10, which is formed of a plurality of material layers stacked on a substrate, a barrier wall 20, which is formed on the base plate 10 to define an ink chamber 22, and a nozzle plate 30, which is formed on the barrier wall 20. The ink chamber 22 is filled with ink. A heater (13 of FIG. 2), which heats ink and generates bubbles, is provided under the ink chamber 22. An ink passage 24 is a path along which ink is supplied into the ink chamber 22. The ink passage 24 provides flow communication from an ink reservoir (not shown). A plurality of nozzles 32, through which ink is ejected, is formed such that one of the plurality of nozzles is formed at a predetermined position to face the ink chamber 22.
The vertical structure of the ink-jet printhead described above will now be described with reference to FIG. 2. Referring to FIG. 2, an insulating layer 12 for insulating the heater 13 from a substrate 11 is formed on the substrate 11, which is formed of silicon. The heater 13, which heats ink in the ink chamber 22 and generates bubbles, is formed on the insulating layer 12. The heater 13 is formed by thinly depositing tantalum nitride (TaN) or a tantalum-aluminum alloy on the insulating layer 12 in a thin film shape. A conductor 14 for applying a current to the heater 13 is formed on the heater 13. The conductor 14 is formed of a material having high conductivity, such as aluminum or an aluminum alloy.
A passivation layer 15, which is formed on the heater 13 and the conductor 14, prevents the heater 13 and the conductor 14 from being oxidized or directly contacting ink. The passivation layer 15 is formed by depositing a silicon nitride layer on the heater 13 and the conductor 14. An anti-cavitation layer 16 is formed on a predetermined portion of the passivation layer 15, on which the ink chamber 22 is to be formed.
The barrier wall 20, which defines the ink chamber 22, is stacked on the base plate 10. The nozzle plate 30, in which the nozzles 32 are formed, is stacked on the barrier wall 20.
FIG. 3 illustrates a variation of a position of a meniscus with respect to time in response to application of a conventional driving signal to an ink-jet printhead. Referring to FIG. 3, when a driving pulse is applied to a heater, bubbles are generated in ink near the heater and continuously expand. Due to this expansion, pressure is applied to ink filling an ink chamber such that ink is ejected through a nozzle. Once ink is ejected, a position of a meniscus on the surface of the ink in the ink chamber gradually stabilizes, but still slightly fluctuates. Before the meniscus is completely damped down, the driving pulse for ejecting ink is applied again to the heater. However, if the meniscus is yet to subside sufficiently, the ejection of ink may not be performed normally.
FIG. 4 is a photograph of an ink droplet ejected from an ink-jet printhead when a meniscus on the surface of ink in an ink chamber is in a stable state. FIG. 5 is a photograph of an ink droplet ejected from an ink-jet printhead when a meniscus on a surface of ink in an ink chamber is in an unstable state. Referring to FIGS. 4 and 5, the tendency of an ink droplet ejected from the ink-jet printhead to travel straight is more distinctively shown in FIG. 4 than in FIG. 5.
In short, ink droplets ejected from an ink-jet printhead are less likely to travel straight, i.e., perpendicular to the upper surface of the printhead, when the ink meniscus is in an unstable state as compared to a state in which the ink meniscus is in a stable state.
Several conventional methods for improving the performance of an ink-jet printhead by modifying a driving signal so that ink droplets can be more efficiently ejected have been proposed. These conventional methods, however, can only thermodynamically improve the performance of an ink-jet printhead by modifying a driving signal or by increasing a temperature of the ink with the use of a pre-pulse before the ink is ejected. Thus far, methods for hydromechanically improving the performance of an ink-jet printhead by modifying a driving signal have not yet been suggested.